Creating a scene with huge amount of geometry
Try using dupliverts:



It's not really that pretty, but this is just a demo.. Rendered in 6 minutes


Create a very subdivided plane, then delete only edges and face so that you are left with just vertices.
Add a cube and parent it to the plane.
Add a displace modifier on the plane, and setup a texture to use a displacement.
Enable dupliverts on the plane.